1. Field of the Invention
The present invention generally relates to a memory cell structure, and in particularly, to a magnetic memory cell structure with thermal assistant.
2. Description of Related Art
Magnetic random access memory (MRAM) has the advantages of non-volatility, high density, high read/write speed, radiation resistance, and so on. When writing data, usually, two current lines (for example, a write bit line (WBL) and a write word line (WWL) are used to sense the cells selected from the intersections of magnetic fields, and the magnetoresistance value of the magnetic cell is changed by changing the magnetization direction of the magnetic material of the memory layer. When reading the memory data, a sense amplifier provides a current to the selected magnetic memory cells, and reads the magnetoresistance value of the magnetic cells to determine the digital value of the memory data.
The magnetic memory cell is a multi-layered magnetic metal material having a stacked structure, including a soft magnetic layer, a tunnel barrier layer, a hard magnetic layer, and a nonmagnetic conductor. The memory “1” or “0” state is determined by the parallel or antiparallel magnetization direction of the magnetic material at two sides of the tunnel barrier layer.
FIG. 1 shows a basic structure of a magnetic memory cell. Referring to FIG. 1, in order to access a magnetic memory cell, current lines 100, 102, also referred to as, for example, a bit line and a word line according to an operation manner thereof are intersected and applied with proper currents. When the two lines are applied with currents, magnetic fields in two directions are generated, so as to obtain a magnetic field with desired magnitude and direction to be applied on a magnetic memory cell 104. The magnetic memory cell 104 has a stacked structure, including a magnetic pinned layer, having a fixed magnetization or a total magnetic moment in a predetermined direction. Various magnetoresistances are generated by using the difference between angles formed between magnetizations of the magnetic free layer and the magnetic pinned layer, for reading data. Further, in order to write data, a write magnetic field is applied to determine the magnetization of the magnetic free layer without the magnetic field. The data stored in the memory cell may be read by output electrodes 106, 108. The operation details of the magnetic memory are known to those of ordinary skill in the art and will not be described herein.
FIG. 2 shows a memory mechanism of a magnetic memory. In FIG. 2, a magnetic pinned layer 104a has a fixed magnetization moment direction 107. A magnetic free layer 104c is located above the magnetic pinned layer 104a and is isolated from the magnetic pinned layer 104a by a tunnel barrier layer 104b. The magnetic free layer 104c has a magnetization moment direction 108a or 108b. If the magnetization moment direction 107 is parallel to the magnetization moment direction 108a, the generated magnetoresistance represents, for example, data of “0”. If the magnetization moment direction 107 is anti-parallel to the magnetization moment direction 108b, the generated magnetoresistance represents, for example, data of “1”.
The magnetic free layer 104c in FIG. 2 has a single-layered structure, and easily generates data error in operation. U.S. Pat. No. 6,545,906 discloses a free layer having a ferromagnetic/non-magnetic metal/ferromagnetic tri-layered structure instead of the single-layered ferromagnetic material in conjunction with a toggle operation mode to reduce the interference of the adjacent cells when writing data. The free layer has a synthetic antiferreomagnet structure, and the top and the bottom magnetic layer each has a magnetization. FIG. 3 shows a structure of a conventional synthetic antiferromagnetic free layer. As shown in FIG. 3, magnetic metal layers 150, 154 disposed on and below a nonmagnetic metal layer 152, are arranged in an antiparallel manner to form closed magnetic lines of force. A magnetic pinned layer 168 beneath is separated from a magnetic free layer 166 by a tunnel barrier layer (T) 156. The magnetic pinned stacked layer 168 includes a top pinned layer (TP) 158, a nonmagnetic metal layer 160, and a bottom pinned layer (BP) 162. The top pinned layer and the bottom pinned layer have a fixed magnetization. Furthermore, an antiferromagnetic layer 164 is included.
For the magnetic free layer 166 having a tri-layered structure, the bit line BL and the write word line WWL are set at an angle of 45 degrees with respect to the magnetic anisotropic axis of the magnetic free layer 166, the magnetic anisotropic axis direction is the so-called easy axis direction. In this manner, the bit line BL and the write word line WWL may apply a magnetic field at an angle of 45 degrees with respect to the easy axis on the magnetic free layer 166 in sequence, to rotate the magnetization of the magnetic free layer 166. The toggle operation mode can effectively eliminate the interference.
However, since advanced processes are continuously adopted in CMOS technology and the design of high-density MRAM is employed, the magnetic tunnel junction (MTJ) interface of the core elements of MRAM is continuously reduced. As for the toggle MTJ elements, the switching field of the free layer will be continuously increased when the size is reduced. At the same time, due to the demagnetization field effect, the magnetization arrangement at the edges of the free layer is not uniform and cannot be controlled, and thus the initial state of the MTJ switching is inconsistent, resulting in non-uniform switching field. Further, the volume is decreased, and the energy barrier (KuV, where Ku is the magnetic anisotropy constant, and V is the magnetic film volume) stored in the free layer is decreased accordingly. When the energy barrier is decreased about to the same level of the thermal energy (kT), the magnetization direction of the free layer in the MTJ will be switched due to the disturbance of the external thermal energy, causing the loss of the stored data in the application of memory.
In order to improve the thermal stability at small size, another technique of a magnetic memory unit with thermal assistant using magnetic field switching has been developed, capable of further reducing the size of the elements. FIGS. 4A to 4C show a schematic view of a memory cell using a thermal assistant mechanism to apply a magnetic field bias. Referring to FIG. 4A, a magnetic free layer 170 on a tunnel barrier layer 156 has a magnetization marked by arrows. Further, an antiferromagnetic layer 172 is formed on the magnetic free layer 170, and a part of the magnetization of the antiferromagnetic layer 172 has an exchange coupling force with the magnetization of the magnetic free layer 170. Since the antiferromagnetic layer 172 is in an antiferromagnetic state at room temperature, the antiferromagnetic layer 172 has a strong coupling force with the magnetization of the magnetic free layer 170. Referring to FIG. 4B, when the antiferromagnetic layer 172 is above the Neel temperature, it becomes a paramagnetic state; and the interactive coupling function with the magnetic free layer 170 disappears. Referring to FIG. 4C, the magnetization of the magnetic free layer 170 is easily switched, thus recording as “0”, “1.” When the temperature is decreased to the room temperature, the characteristics of the antiferromagnetic layer 172 appears again, and the magnetization of the magnetic free layer 170 is fixed.
FIG. 5A is a schematic view of an operating circuit. A magnetic memory cell 180 in FIGS. 4A to 4C is coupled to the words line (WL) and the bit line (BL). The word line controls a switch transistor 181 to be connected to a ground voltage. For example, the antiferromagnetic layer 172 is changed to be paramagnetic by conducting a current Iheat to the word line. And then, a magnetic field for changing the magnetization of the magnetic free layer 170 is generated by conducting a current Ifield to the bit line. Thereafter, the supply of the current Iheat stops, and the temperature returns to the room temperature. FIG. 5B is a schematic view illustrating a relationship between an exchange magnetic bias strength and a temperature. Referring to FIG. 5B, due to the exchange magnetic bias generated by the interactive coupling force, the switching field required by the free layer for switching must be up to 180 Oe, thus the thermal stability is greatly improved. Accordingly, when writing, as long as the temperature is raised to 150° C. or above, the field required for switching is lower than 20 Oe.
Although the prior art can achieve the stability of temperature, the material for providing the exchange field is a horizontal antiferromagnetic material, so the miniaturization of the dimension is limited.
Accordingly, another memory cell structure is provided in the prior art. FIG. 6 shows a structure of another conventional memory cell with thermal assistant. Referring to FIG. 6(a) of FIG. 6, the magnetization of a magnetic pinned layer 182 is perpendicular to the horizontal direction through the formulation of the ingredients of the magnetic material. A perpendicular magnetic free layer 186 is isolated from the magnetic pinned layer 182 by a barrier layer 184, and the magnetization of the perpendicular magnetic free layer 186 is upwardly vertical to the horizontal direction. A heater layer 188 is disposed on the perpendicular magnetic free layer 186. Referring to FIG. 6(b), the temperature of the perpendicular magnetic free layer 186 is raised to a predetermined high temperature by the heater layer 188, so as to change to the paramagnetic state. Referring to FIG. 6(c), a magnetic field H is applied at the high temperature, and the direction of the magnetic field H is reverse to the original magnetization of the perpendicular magnetic free layer 186. The paramagnetic state is likely to generate a magnetization in the same direction of the magnetic field H. Referring to FIG. 6(d), when the temperature is decreased to the room temperature, the magnetization of the perpendicular magnetic free layer 186 is changed to be downward. FIG. 6(a) and FIG. 6(d) show two memory states, which is capable of memorizing one-bit data.
However, as the magnetoresistance (MR) of the perpendicular MTJ element of such a design is too small, and the element is required to have a vertical field design, the density of the memory cannot be effectively improved. Therefore, at present, the MR value of the structure is required to be improved and the size of the structure is required to be reduced.